Control of fluid flow through centrifugal pumps

ABSTRACT

The flow through a steam power plant recirculation circuit is increased and made variable by injection of a certain amount of boiler feed water into the suction side of a centrifugal pump in the recirculation circuit thereby increasing the density of the fluid being pumped and correspondingly increasing the effective pumping pressure in the recirculation circuit.

O United States Patent [151 3,640,251

Ferguson Feb. 8, 197 2 [54] CONTROL OF FLUID FLOW THROUGH 2,277,1003/1942 Hartmann ..55/201 CENTRIFUGAL PUMPS 2,495,525 1/1950 Karassik I22/406 2,662,507 l2/l953 Loumiet et al ..l22/448 Inventor: JeremiahFerguson, Northboro, Mass- 2,679,83l 6/1954 Henkel 1 22/386 [73]Assignee. 113:; Stoker Corporation, Worcester, Prima'y Emminer xennethwp g AttorneyWard, McElhannon, Brooks & Fitzpatrick [22] Filed: Aug. 28,[9'70 [21] Appl. No.: 67,772 [57] ABSTRACT The flow through a steampower plant recirculation circuit is increased and made variable byinjection of a certain amount [52] "122/406 S of boiler feed water intothe suction side of a centrifugal pump [51] Int. Cl ..F22b 35/12 in therecirculation circuit thereby increasing the density of [58] Field ofSearch 1 22/406 R, 406 S, 406 ST the fluid being pumped andcorrespondingly increasing the effective pumping pressure in therecirculation circuit. [56] References Cited 19 Claims, 4 DrawingFigures UNITED STATES PATENTS 1,908,265 5/1933 Lucke ..l22/406 CONTROLOF FLUID FLOW THROUGH CENTRIFUGAL PUMPS This invention relates to thecontrol of fluid flow and more particularly it concerns novelarrangements for controlling the fluid delivery rate of constant speedcentrifugal pumps.

A particularly advantageous application of the present invention occursin the operation of steam generators, especially those which producesupercritical steam for use in power plants. In steam power plants aworking fluid, such as water, is converted to steam by heating in aboiler or steam generator. The steam is then expanded and reduced inpressure in an engine or turbine and the exhaust steam from the turbineis thereafter cooled and converted to liquid form in a condenser. Theliquid is then pumped at high pressure and is returned to the steamgenerator or boiler for reheating and reconversion to steam.

Supercritical boilers are those which heat the working fluid to atemperature at which the phase of the fluid is no longer responsive topressure. That is, the fluid at the supercritical temperature cannot beliquefied no matter what pressure is applied to it. Generally, operationof steam power plants above supercritical temperatures will providemoreefficient and effective operation since the working fluid at suchtemperatures is capable of transferring substantially greater amounts ofheat energy.. There are however a number of problems that must beaccommodated in the design and construction of equipment capable ofhandling working fluids, such as steam, in the supercritical range.

' One requirement of steam generators which operate at supercriticalsteam temperatures is that the fluid flow through the interior conduitsystem of the boiler must be maintained at high velocity irrespective ofthe amount of steam being delivered to the turbine or engine. If thisinternal flow rate should decrease appreciably, the heat transfercharacteristics of the various internal components would deteriorate anddangerous overheating would occur.

In general, a high fluid flow rate through steam generators is achievedby means of a recirculation system which bypasses the engine or turbineand the condenser. Thus the total flow through the steam generator ismade up of two flows, namely: a through flow, which passes from thesteam generator to the engine or turbine then through the condenser andback to the steam generator; and a recirculation flow, which also passesfrom the steam generator but which bypasses the engine or turbine andcondenser and is recirculated back into the steam. generator with thethrough flow from the condenser. If the engine or turbine load shoulddecrease thereby decreasing the through flow, the combined flow throughthe steam generator will also decrease. Accordingly the recirculationflow should be sufficient to maintain an adequate total or combined flowthrough the steam generator under such decreased load conditions.

The recirculation flow is maintained by means of centrifugal pumpsdriven by constant speed motors; and in many instances these pumps arelocated in the recirculation line so that they will not have to handlethe combined flows.

The recirculation pumping requirements in steam generators of largecapacity have been difficult to fulfill. This is because the centrifugalpump in the recirculation line must act upon a working fluid, namelysupercritical fluid of rather low density; and as a result the pump isincapable of generating sufficient pressure to force fluid through therecirculation path at large mass flow rates. I

As steam power plants increase in size the requirements of therecirculation pumps also increase and in fact they may reach a pointwhere the cost of a pump to handle the recirculation flow may become farout of proportion to the overall cost of the plant. One reason for thisis that the recirculating fluid, being of relatively low density must bepumped at very large volume rates to achieve a given mass rate of flow.Pump sizes therefore have to be quite large. In addition, centrifugalpumps are only capable of producing a pressure difference directlyproportional to the density of the fluid being pumped;

and since this density is low the effective pumping forces are also low.

The present invention overcomes the above described difficulties of theprior art. With the present invention it is possible to vary or toincrease the pumping capability and capacity of a fixed speedcentrifugal type pump. It is also possible, with the present inventionto achieve satisfactory recirculation in large steam power plants withmuch smaller centrifugal pumps than has heretofore been necessary.

According to the present invention advantage is taken of the principlethat the pressure difference generated by a centrifugal pump isproportional to the density of the fluid being pumped. Thus the presentinvention provides for an increase in the densityof the fluid passingthrough the pump. In this way the pumping pressure is increased andfluid is forced by the pump around the recirculation path at anincreased mass rate of flow.

As illustratively embodied, the present invention obtains an increase inthe density of the recirculation fluid in a steam generator by injectionof a certain amount of the boiler feed or through flow into therecirculation flow at a location upstream of the recirculation pump butin the recirculation path. The boiler feed flow being of higher densitythan the recirculation flow, mixes with it to produce a combined flow ofincreased density at the recirculation pump input. The pump in turnproduces a greater pressure differential or pumping force and therebycauses a greater mass flow rate of recirculating fluid.

Various further and more specific objects, features and ad-,

vantages of the invention will appear from the description given below,taken in connection with the accompanying drawings, illustrating by wayof example a preferred form of the invention.

In the drawings:

FIG. 1 is a diagrammatic view of a steam power plant in which thepresent is embodied;

FIG. 2 is a graph illustrating typical pump head characteristics of arecirculation pump used in the power plant of FIG. 1;

FIG. 3 is a diagrammatic view of a centrifugal pump used forrecirculation in the power plant of FIG. 1; and

FIG. 4 is a series of graphs useful in explaining the operation of therecirculation control obtained in the power plant of FIG. 1.

The steam power plant represented in FIG. 1 includes a steam generatorunit 10 having a boiler section 12 and an economizer section 14. Thesteam generator unit supplies hot fluid, e.g., at supercriticaltemperatures through an outlet connection 16. The fluid passes from theconnection 16 to a recirculation bypass 18 where the fluid is dividedinto a through flow and a recirculation flow.

The through flow passes along a through flow line 20 to a superheaterand hence to a turbine 22 or other utilization device where the heatcontent of the steam is utilized as by expansion and conversion tomechanical energy. After passing through the turbine and any other steamutilization devices the exhaust steam passes through an exhaust line 24to a condenser 26 where the steam is further cooled and condensed toliquid form. The liquid output or feedwater, from the condenser thenpasses to a feed pump 28 which forces the liquid at high pressure backinto the economizer section 14 of the steam generator 10. The feedwaterreceives heat in the economizer from furnace exhaust gases, and isbrought up to a temperature for injection into the boiler section 12.Upon exiting from the economizer section 14, the preheated feedwaterpasses into a mixer 30 where it is mixed with recirculation flow. Theresulting combined flow passes through a downcomer 32 and back throughthe boiler section l2.

The recirculation flow, which is divided from the combined flow at therecirculation bypass 18, passes along a recirculation line 34 andthrough a recirculation pump 36. The output of the recirculation pump 36is directed to the mixer 30 where it is combined with the preheatedthrough flow for return to the boiler section 12.

Because of the recirculation flow the combined flow which passes throughthe boiler section 12 is increased. Moreover, by maintaining a highrecirculation flow rate, it is possible to provide a substantialcombined flow through the boiler section 12 even at low load conditionswhen only a relatively small through flow is called for.

It is important, especially in supercritical boilers, to maintain a highfluid flow rate through the boiler section even at low load conditions.This is because a high heat transfer is required internally of theboiler to prevent burning and other damage to its structure; and theheat transfer characteristics along most internal boiler surfaces are afunction of the fluid flow rate along them.

The limiting aspect of a boiler recirculation system is the pump whichdrives the recirculation flow. These pumps, for reasons of cost,operating life, maintenance characteristics and other reasons, are ofthe fixed speed, centrifugal type.

While in some boiler recirculation systems the pump providing therecirculation flow drive is not exclusively in the recirculation flowpath, the present description will be directed to situations where, asillustrated in FIG. 1, the recirculation pump 36 is arranged exclusivelyin the recirculation flow path.

The recirculation pump 36 must handle high temperature compressiblefluids of low specific density. Because of this it becomes difficult toachieve a large mass rate of flow for a given size pump. FIG. 2 shows atypical characteristic operating curve (a) for a fixed speed centrifugalpump where head (in feet) is plotted against delivery (in gallons perminute). As can be seen the curve rises in the low delivery rate regionsand then falls off gradually as delivery rate increases. The shape ofthis curve is a function, primarily, of the pump blade design; and itindicates that as the pump blades (moving at constant speed) impinge onan incoming fluid they produce different kinetic effects on the fluidwhere the fluid enters the pump at different speeds.

The head produced by a centrifugal pump corresponds to the height towhich the pump will lift an element of the fluid being pumped. As shownin diagrammatic fashion in FIG. 3 an element (e) of some fluid isimpinged upon by a vane 50 mounted to spin with a rotor 52 within a pumpcasing 54. This impingement throws the element upwardly and out throughan outlet 56 to a height H. Assuming that at the point of impingementthe blade velocity is fully transmitted to the element, then the elementshould rise to the same height H irrespective of its density. Thus thehead produced by a centrifugal pump is independent of the density of thefluid which it pumps.

The head produced by a centrifugal pump can be converted to pressure andin such conversion the density of the fluid being pumped becomes afactor. Thus where a fixed speed centrifugal pump produces the same headfor two fluids of different densities, each fluid, because of itsparticular density, will have a different pressure producing momentum.

FIG. 4 shows at (b) and (c) the pressure curves (in pounds per squareinch) corresponding to the head curve (a) of FIG. 2, for fluids ofdifferent density. As can be seen, where a greater density fluid ispumped to the same head the pressure corresponding to its momentum isgreater than that of the lesser density fluid.

Also shown in FIG. 4 is a stylized curve (d) representative of theoverall pressure losses throughout a fluid flow system (such as therecirculation portion of the system of FIG. 1). In most fluid flowsystems pressure losses are due to the viscous drag produced by themechanical elements of the system, such as piping etc., on the flowingfluid; and since viscous drag increases with increased relativevelocity, the pressure losses are greater at high flow rates.Accordingly, as shown in FIG. 4, the pressure loss curve (d) rises athigher flow rates.

As indicated by the vertical dashed lines (f) and (g) in FIG. 4, theresulting flow through the system is at the intersection of the pumppressure curve (b) or (c) with the pressure loss curve (d). It can beseen from this that in a given fluid flow system where the fluid isdriven by a fixed speed centrifugal pump, the flow rate through thesystem can be increased by increasing the density of the fluid whichpasses through the pump.

Reverting now to FIG. 1 it will be seen that a bypass 38 is interposedbetween the outlet from the economizer l4 and the mixer 30; and thisbypass directs a portion of the preheated feedwater through a bypassline 40, through a valve 42 to a junction 44 where it combines with therecirculation flow in the recirculation line 34 near the inlet orsection side of the recirculation pump 36.

By providing this bypass, a certain amount of relatively cool highdensity boiler feedwater is mixed with the hot recirculation fluid inthe recirculation line 34. This mixing action serves to change thecharacteristics of the fluid so that for a given volume, the weight ofthe fluid is greatly increased. Because the pump 36 now operates on ahigher density fluid its pressure characteristic changes, as from curve(c) to curve (d) in FIG. 4 and the resulting recirculation flow rate iscorrespondingly increased. The valve 42 can be adjusted to control theamount of feedwater admitted to the recirculation flow and in thismanner the delivery characteristics of the pump 36 can be adjusted. Thuseven though the pump 36 operates at constant speed, an effectiveadjustability of the recirculation flow produced by this pump may beobtained by a simple valve control. This adjustment can be controlled inaccordance with load variations so that depending on the amount of steamdemanded in the through flow circuit, including the turbine andcondenser, the rate of recirculation through the recirculation circuitincluding the recirculation flow line 34 and the pump 36 can becorrespondingly adjusted to maintain a proper combined flow through theboiler 12 for protection thereof.

Having described the invention with particularity with reference to thepreferred embodiment of the same, and having referred to some of thepossible modifications thereof, it will be obvious to those skilled inthe art, after understanding the invention, that other changes andmodifications may be made therein without departing from the spirit andscope of the invention; and the appended claims are intended to coversuch changes and modifications as are within the scope of the invention.

What is claimed is:

1. A method for controlling the delivery ofa given fluid by acentrifugal pump, said method comprising the steps of operating saidpump, said method comprising the steps of operating said pump to deliversaid compressible fluid, and during such operation mixing with saidfluid, upstream of said pump, a second fluid of a different density thansaid given fluid thereby to provide a fluid mixture of a differentdensity than said given fluid, which mixture passes through said pump toproduce a changed pressure difference across said pump and acorresponding change in delivery of said pump.

2. A method according to claim 1 where said second fluid is of a greaterdensity than said first fluid.

3. A method according to claim 1 wherein the amount of said second fluidis varied to change the delivery of said pump.

4. A method according to claim 1 wherein said first fluid is steam froma steam generator and wherein said second fluid is feedwater for saidgenerator.

5. A method for controlling the recirculation flow in a steam powerplant of the type having a steam recirculation circuit including acentrifugal recirculation pump to maintain high internal flow velocitiesat low load demands, said method comprising the step of injecting intothe inlet side of said recirculation pump feedwater of higher densitythan the recirculation steam in said recirculation circuit.

6. A method according to claim 5 wherein said recirculation circuit isconnected from the steam output of a steam generator to a mixing devicefor mixing the recirculation steam with boiler feedwater which isdirected back into said steam generator.

7. A method according to claim 6 wherein said feedwater is obtained fromupstream of said mixing device.

8. A method according to claim 7 wherein the amount of feedwaterinjected into said inlet side of said pump is controlled by a valve.

9. A method according to claim 8 wherein said valve is controlled tovary the amount of recirculation in accordance with the load demand onsaid steam power plant to maintain flow through the steam generatorportion thereof.

10. Apparatus for controlling the delivery of a centrifugal pump, saidapparatus comprising pump input conduit means connected to the inputside of said pump, pump output conduit means connected to the outputside of said pump, a first fluid supply means connected to supply fluidof one density to said pump, input conduit means and fluid injectionmeans connected to inject fluid of a different density into the inletside of said pump.

11. Apparatus according to claim 10 further including a recirculationcircuit interconnecting the output and input sides of said pump.

12. Apparatus according to claim 11 wherein at least a portion of saidrecirculation circuit is shared with a second fluid circuit whichsupplies to said recirculation circuit said fluid of a differentdensity.

13. Apparatus according to claim 12 wherein a further conduit isconnected to direct a portion of said fluid of a difierent density fromsaid second fluid circuit into the inlet side of said pump.

14. Apparatus according to claim 13 wherein said further conduitincludes a valve.

15. In combination with a steam generator, steam outlet means from saidgenerator, feedwater inlet means to said generator, a recirculationcircuit including a bypass connected to said steam outlet means, amixing device connected to said feedwater inlet means and recirculationconduit means including a centrifugal recirculation pump connectedbetween said bypass and said mixing device and feedwater injection meansconnected to receive boiler feedwater from upstream of said mixingdevice and to direct same into the inlet side of said centrifugalrecirculation pump.

16. A combination according to claim 15 wherein said steam generator isof the supercritical type.

17. A combination according to claim 16 wherein said mixing device isarranged to receive preheated feedwater from an economizer portion ofsaid steam generator.

18. A combination according to claim 17 wherein said feedwater injectionmeans comprises a conduit interconnecting the feedwater output from saideconomizer portion and said inlet side of said pump.

19. A combination according to claim 18 wherein said feedwater injectionmeans includes a valve to control the ratio of feedwater andrecirculation steam supplied to said pump.

1. A method for controlling the delivery of a given fluid by acentrifugal pump, said method comprising the steps of operating saidpump to deliver said compressible fluid, and during such operationmixing with said fluid, upstream of said pump, a second fluid of adifferent density than said given fluid thereby to provide a fluidmixture of a different density than said given fluid, which mixturepasses through said pump to produce a changed pressure difference acrosssaid pump and a corresponding change in delivery of said pump.
 2. Amethod according to claim 1 where said second fluid is of a greaterdensity than said first fluid.
 3. A method according to claim 1 whereinthe amount of said second fluid is varied to change the delivery of saidpump.
 4. A method according to claim 1 wherein said first fluid is steamfrom a steam generator and wherein said second fluid is feedwater forsaid generator.
 5. A method for controlling the recirculation flow in asteam power plant of the type having a steam recirculation circuitincluding a centrifugal recirculation pump to maintain high internalflow velocities at low load demands, said method comprising the step ofinjecting into the inlet side of said recirculation pump feedwater ofhigher density than the recirculation steam in said recirculationcircuit.
 6. A method according to claim 5 wherein said recirculationcircuit is connected from the steam output of a steam generator to amixing device for mixing the recirculation steam with boiler feedwaterwhich is directed back into said steam generator.
 7. A method accordingto claim 6 wherein said feedwater is obtained from upstream of saidmixing device.
 8. A method according to claim 7 wherein the amount offeedwater injected into said inlet side of said pump is controlled by avalve.
 9. A method according to claim 8 wherein said valve is controlledto vary the amount of recirculation in accordance with the load demandon said steam power plant to maintain flow through the steam generatorportion thereof.
 10. Apparatus for controlling the delivery of acentrifugal pump, said apparatus comprising pump input conduit meansconnected to the input side of said pump, pump output conduit meansconnected to the output side of said pump, a first fluid supply meansconnected to supply fluid of one density to said pump, input conduitmeans and fluid injection means connected to inject fluid of a differentdensity into the inlet side of said pump.
 11. Apparatus according toclaim 10 further including a recirculation circuit interconnecting theoutput and input sides of said pump.
 12. Apparatus according to claim 11wherein at least a portion of said recirculation circuit is shared witha second fluid circuit which supplies to said recirculation circuit saidfluid of a different density.
 13. Apparatus according to claim 12wherein a further conduit is connected to direct a portion of said fluidof a different density from said second fluid circuit into the inletside of said pump.
 14. Apparatus according to claim 13 wherein saidfurther conduit includes a valve.
 15. In combination with a steamgenerator, steam outlet means from said generator, feedwater inlet meansto said generator, a recirculation circuit including a bypass connectedto said steam outlet means, a mixing device connected to said feedwaterinlet means and recirculation conduit means including a centrifugalrecirculation pump connected between said bypass and said mixing deviceand feedwater injection means connected to receive boiler feedwater fromupstream of said mixing device and to direct same into the inlet side ofsaid centrifugal recirculation pump.
 16. A combination according toclaim 15 wherein said steam generator is of the supercritical type. 17.A combination according to claim 16 wherein said mixing device isarranged to receive preheated feedwater from an economizer portion ofsaid steam generator.
 18. A combination accOrding to claim 17 whereinsaid feedwater injection means comprises a conduit interconnecting thefeedwater output from said economizer portion and said inlet side ofsaid pump.
 19. A combination according to claim 18 wherein saidfeedwater injection means includes a valve to control the ratio offeedwater and recirculation steam supplied to said pump.